1. Field of the Invention
The invention relates to an image fixing apparatus for fixing a toner image on a sheet and an image forming apparatus provided with the same.
2. Description of Related Art
The following is known as an image fixing apparatus having a plurality of fixing means.
In Japanese Patent Application Laid-open No. H04-245275, it is disclosed that provision is made of two sets of fixing roller groups each comprising a pair of heating roller and pressure roller urged against each other, and design is made such that recording paper to which a toner image has been transferred is passed through first stage and second stage pressure contact portions, whereby gloss is given to the toner image after fixed, and it is described that a glossy image can be obtained by the use of two sets of fixing devices.
In Japanese Patent Application Laid-open No. H05-002352, it is disclosed that provision is made of two sets of fixing roller apparatuses each comprising a pair of heating roller and pressure roller urged against each other, and the two sets of fixing roller apparatuses are disposed so that each of the nip portions of the two sets of fixing roller apparatuses may be along a substantially straight conveying path. Also, the pressure force of each of the pressure rollers against the heating rollers of the two sets of fixing roller apparatuses is made small to such a degree as not to produce wrinkles, and a fixing property is maintained good by fixing twice correspondingly to a reduction in the fixing property caused by the pressure force being made small. In this patent application, it is disclosed that by the above-described construction, a recording medium leaves the nip position of a first fixing roller and is guided straight to the nip position of a second fixing roller, whereby the leading edge of the recording medium can be conveyed to the nip without striking against the second fixing roller, and such a shock as acts on a portion of the recording medium nipped by the first fixing roller is eliminated.
Japanese Patent Application Laid-open No. 2000-075710 discloses a fixing apparatus in which the conveying speed of a recording material by first fixing means provided on an upstream side is set higher than the conveying speed of the recording material by second fixing means provided on a downstream side so that unnecessary tension may not be applied to the recording material between the two fixing means, and which can prevent the occurrence of image misregister and the wrinkles of the paper.
There have been proposed examples of the image forming apparatus which output an image of high gloss by the use of two fixing devices, as in the above-described conventional examples, and there have been disclosed many examples in which such a problem as image misregister due to the wrinkles or shock of the recording material is caused by the two fixing devices.
In recent years, in image forming apparatuses such as copying machines and printers, there has been rising a requirement for more various types of recording materials than before such as thin sheets to thick sheets that unfixed toner images on the recording materials be fixed. Also, for the various recording materials, it has become necessary to maintain high productivity without lowering the throughput.
In the aforedescribed conventional examples, however, there has been a problem which cannot be completely solved even by the fixing apparatuses which attempt to solve such a problem as the image misregister due to the shock of the recording material as noted above. That is, in a case where the conveying speed of the recording material by the first fixing means provided on the upstream side is set higher than the conveying speed of the recording material by the second fixing means provided on the downstream side so that unnecessary tension may not be applied to the recording material by the two fixing means, if the recording material is a thin sheet, the recording material is flexed between the two fixing means to thereby give birth to an effect, but there has been the problem that if the recording material is a thick sheet, it is difficult for the recording material to be flexed and the shock with which the recording material dashes into the second fixing means is transmitted to the first fixing means to thereby give rise to image misregister.
In Japanese Patent Application Laid-open No. 2002-351237, it is disclosed that in an image forming apparatus having two fixing means, when a recording material is being conveyed by both of upstream side fixing means and downstream side fixing means, a loop amount form in the recording material is detected, and on the basis of the result of the detection, the conveying speeds of the two fixing means are determined. This, however, requires a mechanism for detecting the loop amount and a construction for changing the conveying speeds on the basis of the loop amount, and the construction of the apparatus becomes complicated and the cost of the apparatus is increased.
Also, in Japanese Patent Application Laid-open No. S63-116181, it is disclosed that two pairs of fixing rollers are provided and the speed of the fixing roller on the downstream side is made higher than that of the fixing roller on the upstream side, but there is the possibility that when a recording material is conveyed by both of the two fixing rollers, tension may be applied to the recording material to thereby disturb a toner image on the recording material, thus causing faulty fixing.